1. Field of the Invention
The present invention is directed to a physiological recording device, or other types of sensors to detect a biopotential, and more particularly, a physiological recording electrode that can be used without skin preparation or the use of electrolytic gels. The invention is further directed to a variety of configurations of structures on the physiological recording electrode's lower surface.
2. Technical Background
Electrodes for measuring biopotential are used extensively in modern clinical and biomedical applications. These applications encompass numerous physiological tests including electrocardiography (ECG), electroencephalography (EEG), electrical impedance tomography (EIT), electromyography (EMG) and electro-oculography (EOG). The electrodes for these types of physiological tests function as a transducer by transforming the electric potentials or biopotentials within the body into an electric voltage that can be measured by conventional measurement and recording devices.
In general, most commercial physiological electrodes for these applications today are placed on the surface of the skin. Because of this it is important to understand the anatomy of the skin to understand the problems encountered with these electrodes. The skin is a layered structure, which consists of the epidermis and the dermis. The dermis contains the vascular and nervous components. Further it is the part of the skin where pain has its origins. The epidermis is an important layer in the electrode/skin interface. The epidermis consists of a number of layers as shown schematically in FIG. 1. These layers consist of:
a) Stratum basale or stratum germinativum, which contains living basal cells that grow and divide, eventually migrating into the other layers of the epidermis;
b) Stratum spinosum, which contains living cells that have migrated from the stratum basale. The early stages of desmosomes can be found in this layer;
c) Stratum granulosum, which contains cells with many desmosomal connections, forms a waterproof barrier that prevents fluid loss from the body;
d) Stratum lucidum, which is a transition layer between the stratum granulosum and the stratum corneum. It is thickest, in high friction areas such as the palms and the soles of the feet; and
e) Stratum corneum, which is the outer layer, contains dry, dead cells, flattened to form a relatively continuous thin outer membrane of relatively continuous thin outer membrane of skin. The deeper cells of this layer still retain the desmosomal connections, but as they are pushed toward the surface by newly formed cells in the underlying layers, the junctions gradually break and the cells are lost.
The stratum corneum is the primary source of high electrical impedance. This is because dead tissue has different electrical characteristics from live tissue, and has much higher electrical impedance. Thus, this layer dramatically influences the biopotential measurements. The stratum corneum is estimated to be typically approximately one tenth the thickness of the epidermis except for the palms of the hand and the foot where this layer is much thicker. The stratum corneum, further, is very thin and uniform in most regions of the body surface ranging from 13-15 μm with a maximum of about 20 μm. If the high impedance results from the stratum corneum can be reduced, a more stable electrode will result. Therefore with existing physiological electrodes the skin must be prepared prior to application when lower impedance is required.
The most common electrode preparation methods to improve the electrode signal and avoid the high impedance effects of the stratum corneum are: 1) shaving the hair from the skin; and 2a) abrading the stratum corneum and/or 2b) using an electrolytic gel. Electrodes having or using an electrolytic gel or fluid are often referred to as—“wet” electrodes. Hair is shaved from the skin to improve the contact between the electrodes and the skin surface. The goal of the abrasion of the stratum corneum is to reduce the thickness of (or remove) the stratum corneum (and therefore its electrically insulating characteristics). Drawbacks of abrading the skin are that the abraded area regenerates dead cells fairly quickly (resulting in a limited time period for using the electrode), and if the abrasion is too deep the person can experience pain. Additionally, electrolytic gels or fluids may be applied to abraded surface to enhance the contact. Alternatively, electrolytic gels or fluids can be applied to the surface of the skin directly. The electrolytic gel, having a high concentration of conductive ions, diffuses into the stratum corneum and improves its conductivity. Drawbacks observed with the use of electrolytic gels or fluids involve the change of conductivity with time as the gels dry, discomfort (an itching sensation) at the patient's skin as a result of the gels drying, and the possibility of a rash due to an allergic reaction to the electrolytic gels.
Further drawbacks of “wet” electrodes include skin preparation and stabilization of the electrode with respect to the skin surface. This is because movement of the electrode on the surface of the skin causes the thickness of the electrolytic layer (formed by the electrolytic gels or fluids) to change resulting in false variation in the measured biopotential commonly referred to as motion artifacts. Some electrode designs have an adhesive backing to reduce the movement of the electrode on the skin surface. However, neither of these features eliminates completely the movement of the electrode with respect to the subject's skin. Another drawback is the length of time required to prepare the skin and apply the electrolytic gels or fluids prior to measurement of the biopotentials.
A less common type of physiological electrode is a non-polarizable “dry” electrode. These ceramic, high sodium ion conducting electrodes do not need an electrolytic gel before their application. The principal of the measurements from these physiological electrodes is based on a sodium ion exchange between the skin and the electrode. The skin-electrode impedance of these type of electrodes are found to decrease as a function of application time. This is a result of perspiration being produced by the body under the electrode almost immediately after application of the electrode on the skin. Drawbacks again, however, include many of those experienced with “wet” electrodes.
Another less common type of physiological electrode is an active “dry” electrode with an amplifier. Advances in solid-state electronic technology have made it possible to record surface biopotentials utilizing electrodes that can be applied directly to the skin without abrading the skin or using an electrolytic gel. These electrodes are not based on an electrochemical electrode-electrolyte interface. Rather, these electrodes are active and contain a very high impedance-converting amplifier. By incorporating the high impedance-converting amplifier into the electrode, biopotentials can be detected with minimal or no distortion. Although these electrodes offer the advantage of not requiring some of the preparation needed with conventional electrodes, they have certain inherent disadvantages. These electrodes are bulky in size due to the additional electronics and power sources required and they are typically more expensive to produce due to the electronic assembly required. Further, these electrodes also result in motion artifacts due to poor electrode-skin contact similar to electrodes requiring electrolytic gels or fluids.
In view of the foregoing inherent disadvantages with presently available wet and dry electrodes, it has become desirable to develop an electrode that does not require skin preparation or the use of electrolytic gels and overcomes the inherent disadvantages of presently available dry electrodes.